1. Field of the Invention
This invention relates to filter-based methods and systems for measuring angular speed of an object.
2. Background Art
A gyroscope is an angular rate sensor that measures angular speed about a specific axis. Gyroscopes with high reliability and low cost are required in specific applications, such as inertial navigation in the automotive industry. There has been considerable interest in developing micromachined angular rate sensors to satisfy these requirements because of their low cost and small size. To improve the sensitivity of a gyroscope, it is best to match the resonance frequencies in drive and detect modes. However, due to production variations and temperature dependencies, these frequencies tend to deviate from a matched condition.
Vibrating Gyroscopes
The principle of operation for an angular rate sensor is based on the Coriolis effect. In particular, when a moving particle is rotated, Coriolis forces are generated due to conservation of linear momentum. The Coriolis force is given by:
xe2x80x83Fc=2m(xcfx89xv),xe2x80x83xe2x80x83(1),
where m is the mass of the particle, xcfx89 is its rotate rate vector, and v is its linear velocity. The rotation rate vector, denoted by xcfx89, is normal to the plane of rotation. The Coriolis force is generated in the rotating frame.
Due to the cross product in (1), Fcx, the Coriolis force along the x-axis, only depends on the velocity component along the plane of rotation as seen in FIG. 1. Hence, the resulting Coriolis force is directly proportional to the angular rotation of the object 10 around the axis perpendicular to the velocity of the object 10 with Vz, velocity component normal to plane of rotation and Vy, velocity component on plane of rotation. The angular rate can be measured by detecting the magnitude of this force.
In vibratory gyroscopes, the linear velocity component is generated by driving a mechanical structure into resonance vibration. When the structure is rotated, the Coriolis force induces vibration perpendicular to the reference vibration with the same frequency. The reference and Coriolis-induced vibration modes are perpendicular to each other and together display an elliptical motion. The angular rate is measured by sensing the amplitude of the induced oscillation component, which is proportional to the Coriolis force (hence, proportional to the rotation rate).
The amplitude of the Coriolis-induced sense vibration is generally smaller than the reference vibration. To be able to detect the induced vibration, the reference oscillation amplitude should be kept sufficiently large. This is accomplished by operating the structure at resonance. This takes advantage of the quality factor (Q) amplification near the resonance frequency.
For best performance, the reference and induced vibrations should have equal resonance frequencies. However, it is very difficult to match the two frequencies practically. These frequencies depend on several parameters such as mechanical structure and temperature. Mechanical design limitations and temperature shifts will cause mismatch in resonance frequencies. This small mismatch will cause a large variation in the sensitivity of angular rate sensor.
Another disadvantage of designing the sense mode at resonance is the limited operation bandwidth. This is demonstrated by time domain analysis of the prior art system illustrated in FIG. 2. In FIG. 2, a movable mass 12 is supported by anchors 14.
The reference oscillation along the x-axis, indicated at 16, with amplitude Xo and frequency xcfx89x is given as a function of time by:
x(t)=Xo sin xcfx89xt.xe2x80x83xe2x80x83(2) 
The rotation applied around the y-axis, as indicated at 20, induces a Coriolis force along the z-axis, indicated at 18, which is given by:                                                                         F                c                            =                              2                ⁢                m                ⁢                                  xe2x80x83                                ⁢                                                      Ω                    y                                    ⁡                                      (                    t                    )                                                  xc3x97                                                      x                    .                                    ⁡                                      (                    t                    )                                                                                                                                          =                                  2                  ⁢                  m                  ⁢                                      xe2x80x83                                    ⁢                                                            Ω                      y                                        ⁡                                          (                      t                      )                                                        ⁢                                      X                    o                                    ⁢                                      ω                    x                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      ω                    x                                    ⁢                  t                                            ,                                                          (        3        )            
where m is the mass of the structure and xcexa9y(t) is the angular frequency of the rotation as a function of time. In Equation (3), it is seen that the Coriolis force is an amplitude-modulated signal where the carrier frequency is the reference vibration frequency. Thus, the resulting Coriolis force is a dual sideband signal centered on the reference frequency, with a bandwidth twice the bandwidth of the input angular rate. However, if the sense mode is at resonance, because of the sharp response with high Q, the sense oscillation will not have the same bandwidth as the Coriolis force signal. The bandwidth of the detected angular rate will be much smaller than the input rate.
For the reasons mentioned above, the sense mode is in general designed to operate at a much lower frequency than resonance. FIG. 3 shows example drive and sense mode spectra 22 and 24, respectively, for conventional gyroscope designs. Here, the resonance frequency of the sense mode is much higher than the reference frequency, yielding a smoother frequency region for the sense mode. This improves the bandwidth and temperature stability of the sensor, but results in a significant decrease in the sensitivity compared to a gyroscope that would be operated at resonance.
U.S. Pat. No. 5,945,600 to Touge et al. shows a low profile angular rate sensor having a first comb-type drive resonator or oscillator coupled to a second comb-type resonator or oscillator via a movable electrode. The movable electrode detects the vertical movement or deflection of the oscillations along the Z-axis after the input of a Coriolis force. The resonant frequencies of the oscillations in the X and Z directions are the same.
U.S. Pat. No. 5,945,599 to Fujiyoshi et al. discloses a resonance-type angular velocity sensor in which capacitive-type electrodes are used to detect the direction of a Coriolis force. The exciting frequency is adjustable.
U.S. Pat. No. 5,895,850 to Buestgens discloses a micromechanical resonator or a vibration gyrometer in which a pair of spaced resonating masses are joined via a coupling mass.
U.S. Pat. No. 5,604,311 to Kumar et al. discloses a Coriolis effect rotation rate sensor and method including a tuning mechanism for the in-situ tuning of resonant frequencies.
U.S. Pat. No. 5,889,208 to Nose; U.S. Pat. Nos. 5,895,852 and 6,070,463 to Moriya et al.; U.S. Pat. No. 5,918,280 to Gang et al.; U.S. Pat. No. 5,969,225 to Kobayashi; U.S. Pat. No. 5,992,233 to Clark; U.S. Pat. No. 6,044,707 to Kato; and U.S. Pat. No. 6,067,858 to Clark et al. all disclose micromechanical vibratory rate gyroscopes or angular rate sensors per se in which detection along the Z-axis is facilitated.
U.S. Pat. No. 3,839,915 to Schlitt and U.S. Pat. No. 5,197,331 to Oikawa both disclose oscillating angular rate sensors employing electronic filtering capability.
Other relevant U.S. patents include U.S. Pat. Nos. 5,455,547; 4,654,663; 5,604,312; 5,635,638; 5,728,936; 5,955,668; 6,023,972; 6,089,089; and 6,214,243.
An object of the present invention is to provide a filter-based method and system for measuring angular speed of an object which is inexpensive and highly reliable.
Another object of the present invention is to provide a filter-based method and system for measuring angular speed of an object which has increased stability and sensitivity.
In carrying out the above objects and other objects of the present invention, a method for measuring angular speed of an object is provided. The method includes providing a micromechanical filter apparatus including one or more intercoupled micromechanical elements including a first resonator having a first resonance frequency formed on a substrate and having a drive mode response in a drive mode wherein the filter apparatus has a filter response in a sense mode. The method further includes coupling the substrate to the object so that the filter apparatus rotates with the object about a first axis. The method also includes driving the first resonator in the drive mode so that the first resonator vibrates along a second axis at a reference vibration and generates a Coriolis force which causes one of the other elements of the filter apparatus to vibrate along a third axis at an induced vibration. The method further includes sensing the induced vibration in the sense mode to obtain a corresponding output signal which represents the angular speed of the object about the first axis.
The micromechanical elements may include a second resonator having a second resonance frequency wherein the resonance frequencies are substantially the same in the drive and sense modes.
The filter response in the sense mode may have a substantially constant amplitude region for a passband of frequencies including the resonance frequencies. The filter response of the filter apparatus in the sense mode may be substantially constant about the resonance frequencies.
The micromechanical elements may also include a second resonator coupled to the first resonator wherein the first resonator is driven during the step of driving in the drive mode so that the first resonator vibrates along the second axis at the reference vibration and generates the Coriolis force to cause the second resonator to vibrate along the third axis at the induced vibration.
The resonators may be platform, disk or wineglass resonators.
The first resonator may be comb-driven.
The step of sensing may be performed capacitively.
Q-multiplication may be attained in both the drive and sense modes.
The resonators may be polysilicon resonators.
The micromechanical elements may further include a mechanical spring for coupling the resonators together.
The filter apparatus may be a wide passband filter apparatus wherein the filter response is a wide passband filter response.
Further in carrying out the above objects and other objects of the present invention, a system is provided for measuring angular speed of an object. The system includes a substrate and a micromechanical filter apparatus including one or more intercoupled micromechanical elements including a first resonator having a first resonance frequency formed on the substrate and having a drive mode response in a drive mode wherein the filter apparatus has a filter response in a sense mode. The filter apparatus rotates with the object about a first axis when the substrate is coupled to the object and the object is rotated. The system further includes means for driving the first resonator in the drive mode so that the first resonator vibrates along a second axis at a reference vibration and generates a Coriolis force which causes one of the other elements of the filter apparatus to vibrate along a third axis at an induced vibration. The system further includes means for sensing the induced vibration in the sense mode to obtain a corresponding output signal which represents the angular speed of the object about the first axis.
The micromechanical elements may also include a second resonator coupled to the first resonator wherein the first resonator is driven by the means for driving in the drive mode so that the first resonator vibrates along the second axis at the reference vibration and generates the Coriolis force to cause the second resonator to vibrate along the third axis at the induced vibration.
The means for sensing may include a capacitor for capacitively sensing the induced vibration.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.